A clamping device for an information storage disk must satisfy several conditions. First, it must secure the disk firmly enough to prevent slippage when a rotational force is applied to the disk during acceleration or deceleration. The clamping force must also be strong enough to resist any external side shock loads which are imposed on the disk drive. It must avoid distorting the disk. It must be easy to install and remove without damaging the disk or spin motor. It should minimize the amount of particulate debris which is generated during assembly of the disk in the drive. It should be perfectly balanced and should be manufactured from materials that do not outgass or otherwise contaminate the disk.
Most prior art clamps are "axial clamps" which rely on a force parallel to the axis of the drive shaft ("axial force") that is imposed on a flat annular region near the inside edge of the disk. The clamp forces the disk against a flange on the drive motor hub or against a spacer (in a multidisk arrangement). Friction between the disk surface and the clamp or spacer, or the hub flange, provides a force which resists slippage between the disk and its neighboring components in a circumferential or radial direction. The clamping force is typically provided by one or more screws, which are tightened to grip the disk between the clamp or spacer and the hub flange, the latter of which establishes the correct elevation of the disk in relation to the read/write head.
This type of clamp presents several problems. First, the axial clamping force is rarely strong or uniform enough to prevent the disk from shifting off center when the disk drive is subjected to a shock force. Second, the various components normally have different coefficients of thermal expansion, and thus the clamping force may vary as the drive is subjected to different temperatures within its operating range (typically -40.degree. C. to +60.degree. C.). Moreover, the surfaces rarely mate perfectly, and as the temperature varies, one or more points on the disk will tend to stick to the clamp, while other points become free to shift. This can also lead to an eccentric disk. When the disk shifts for whatever reason, the data which have been written on the disk will become eccentric.
The lack of perfect mating between an axial clamp and a disk also causes the clamp to warp the disk into various shapes. Depending upon the exact nature of the mismatch, the disk may be warped into the shape of a cone or a "taco", or relatively short wavelength ripples or bumps may appear in the disk. Any of these deformations may cause problems for current low-flying heads which are typically separated from the surface of the disk by approximately 5 microinches. The length of a typical air bearing surface is approximately 0.1 inches, or 100,000 microinches. Curvature of the disk surface of only 1 microinch in this distance will create an error in the gap between the disk and head of about 0.5 microinches.
In an attempt to overcome these problems, elastomeric or polymeric washers have been placed between the disk and the clamp and hub, or between the disks and spacers in a multidisk stack arrangement. Using these deformable elements can create problems with tolerances, structure and part count, however, and elastomeric materials are likely to be sources of outgassing contamination.
Moreover, as noted above, axial clamps are typically tightened with one or more screws. This process tends to create side loads during installation that force the disks and spacers against one side of the hub, thereby imbalancing the structure. The task of balancing disks that have been mounted in this way is a time-consuming but necessary part of the manufacturing process.